Tribology of Unfilled and Filled Polymeric Surfaces in Refrigerant Environment for Compressor Applications

Abstract

Recent changes in environmental laws requiring the transition from known ozone depleting chlorofluorocarbon (CFC) refrigerants such as R-22 to safer alternative refrigerants such as fluorocarbon based R- 134A and R-410A, have necessitated other changes in the refrigeration systems as well. Refrigeration compressor oils have also been replaced in order to be miscible with the alternative refrigerants. These alternative refrigerants and oils have changed the tribological characteristics of compressor critical contacts, and in some cases have led to an increase in failure rates. Much research has been conducted on the compressor contacts with alternative refrigerants and oils to understand the tribological impacts. Polymers have seen very limited study related to compressor tribocontacts in the presence of refrigerants. Polymers have a self lubricating effect by transferring material to the metal counterface. This is an important group of materials for tribological applications, especially blended polymers, which often have enhanced mechanical and low friction properties. Most of the literature on polymer tribology is conducted at speeds and loads significantly lower than typical compressor conditions. At these low speeds and applied loads, varying degrees of polymer transfer films are reported on the metal counterfaces. It is postulated that coherent transfer films are necessary for reduced wear. The current study looks at the tribological response of polymer/metal contacts in the presence of refrigerant versus ambient air under conditions simulating refrigeration compressors. Ten different polymers are employed as potential compressor bearing materials; four unfilled polymers and six blended polymers. Friction coefficient, wear, and surface topography were evaluated at a 60??C system temperature, 25 psi R-134A atmosphere (or ambient air), 2.4 m/s sliding velocity, and 45 or 225 N applied loads. Polymers were tested against cast iron disks of roughness 0.3 to 0.5 ??m Rq. Experiments conducted in R-134A show slightly favorable friction and wear characteristics to experiments conducted in ambient air. All blended polymers have good tribological characteristics. PEEK and polyimide in both unfilled and blended forms exhibit minimal wear and do not adversely affect the metal disks. These polymers show promise for compressor bearing materials. Representative testing in starved lubricant conditions shows decreased polymer friction and wear. This study also shows that although coherent, uniform films are not produced under compressor-like conditions, as evidenced by scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX), the tested polymers still have favorable tribological properties.Air Conditioning and Refrigeration Project 14